Project Summary It has become increasingly evident that women who are taking combined hormone replacement therapy (CHRT) are at an increased risk of developing breast cancer, however, the potential impact of this form of hormone use on breast cancer mortality has not been thoroughly studied, and, at present our knowledge of this impact (if any) is incomplete. We propose to conduct a population-based nested case-control study of breast cancer deaths from 1990 through 2007 in Saskatchewan, Canada by linking several provincial-level data files, including the population registry, vital statistics registry, Saskatchewan Cancer Registry, Prescription Drug Plan, hospital services, physician services, and the Breast Cancer Screening Program. Each person registered with Saskatchewan Health (a publicly funded health care system) has a unique Health Services Number which enables an individual's records to be linked across these data files. The availability of these data will permit a relatively efficient assessment of the risk of breast cancer death in relation to the duration, recency, and type of CHRT use, both overall and for specific histologic types (ductal and lobular) of breast cancer. PUBLIC HEALTH RELEVANCE: Relevance In recent years epidemiologic studies are reasonably clear in their findings of an increased breast cancer incidence in postmenopausal women who have taken combined hormone replacement therapy (CHRT). However, what is not clear is the degree to which the increase in incidence will translate into an increase in breast cancer mortality - only a single study has compared mortality rates from breast cancer among women without breast cancer who do and do not initiate CHRT - and the results from this study were inconclusive. Because CHRT continues to be widely used (some two million American women are receiving this treatment at the present time), and because CHRT has benefits that have to be weighted against the risks, an assessment of breast cancer mortality in CHRT users represents an important undertaking.